This invention relates to franking machines and in particular to franking machines in which accounting and control functions are carried out by electronic circuits and printing of franking impressions is effected by an electronically controlled digital printing device, for example by a thermal transfer printer.
In known franking machines it is common for the machine to comprise a base unit and a postage metering unit mounted on the base unit. The base unit contains electrical power supplies for the franking machine and means for feeding mail items through the franking machine. The postage metering unit includes electronic circuits, commonly implemented by a microprocessor, to carry out accounting functions and to control various operations to be performed by the franking machine. A keyboard is provided to enable input to the microprocessor of function selecting signals and of data, for example selected values of postage charge. A display device is driven by the microprocessor to echo the input of the keyboard to enable a user to verify that the correct and intended input has been effected and also to display other information to enable the user to ascertain the status of the machine and to otherwise assist the user in using the machine. Non-volatile memory devices are provided to store accounting data relating to use of the machine in carrying out franking of mail items. It is usual for the memories to have a number of registers acting respectively as a descending register to store a value of credit currently available for use in franking mail items with postage charges, an ascending tote register to store an accumulated value of postage charge used in franking mail items, an items count register to store a count of the number of items franked by the machine and a high items register to store a count of the number of items franked with a value of postage charge greater than a predetermined value. The accounting data stored in the memories provides an accounting record of revenue to the postage authority by whom the franking machine is licensed to be used and also provides an accounting record of expenditure by the user in respect of postage charges for mail items franked by the machine and subsequently handled by the postal authority. Accordingly it is necessary that the accounting data is accurately generated and maintained. The postage metering unit also includes a printer for printing franking impressions on the mail items. The printer is controlled by the electronic circuits such that the printer prints an indication in the franking impression of the value of postage charge for which accounting has been carried out. When a value of postage charge is selected by the user, accounting circuits of the meter check the value of credit in the descending register available for use in franking and if there is sufficient credit, the descending register is decremented by the amount of the selected value of postage charge, the ascending register is incremented by the same selected value, the items count is incremented by unity and the printer is operated to print a franking impression, the franking impression including an indication of the selected value of postage charge.
If unauthorised access can be obtained to those parts of the postage metering unit utilised for carrying out accounting functions and for printing, such access could be used to tamper with operation of the accounting circuits or printer or to change data in the accounting records whereby postage charges could be applied fraudulently in franking impressions on mail items without correctly accounting for those charges in the accounting records of the postage metering unit. Accordingly the postage metering unit is contained in a secure housing sealed against unauthorised access. If the seal is broken the postal authority is alerted to unauthorised access to the postage metering unit having occurred. Access is permitted by authorised personnel only and after such authorised access the secure housing is resealed.
A problem arises if there is a failure of any of the components of the postage metering unit because in order to repair the unit, access to the interior of the secure housing is required and such access would expose the critical accounting and control circuits to the possibility of tampering. Accordingly repairs to the postage metering unit may be carried out only in a secure manner by these persons authorised by the postal authority and entrusted with ensuring that the integrity of the accounting records is retained in the course of carrying out the repairs.